"They are brazenly and unforgivably violating my constitutional right and the right of my interlocutors to private communications and telephone conversations," he said, accusing them of "criminal activities".

He said Mr Putin and Mr Surkov were seeking to disrupt an opposition rally planned for 24 December.

To be honest, I am outraged about our taxes being used on able-bodied men invading privacyYevgenia Chirikova, Opposition activist

'Dirty trick'

Appearing together on independent TV channel Dozhd (Rain), Ms Chirikova and Mr Nemtsov said those behind the phone tap had not only failed to split the opposition but had achieved the opposite result.

""Zhenya [Chirikova] and I... decided that we would be even more united in moving hand in hand towards our common goal, which is to rid Russia of the crooks and thieves," said Mr Nemtsov, disparagingly referring to Mr Putin's ruling United Russia party.

Ms Chirikova described the release of the recordings as "an absolutely dirty trick".

"To be honest, I am outraged about our taxes being used on able-bodied men invading privacy," she said.

Our correspondent says it is widely assumed that the Russian security services listen into and record phone calls, and that prominent anti-government figures are targeted.

But he says the fact that six hours of recordings have been given to the website for publication is unusual.

Earlier this month Mr Nemtsov was among at least 250 protesters arrested during protests in Moscow over alleged fraud in the parliamentary election.

Mr Putin's party, United Russia, won but with just under 50% of the vote, a sharp drop in its support.